Transmitters form a significant portion of most communication circuits. As such, they assume a position of prominence in design concerns. With the proliferation of mobile terminals, transmitter design has progressed in leaps and bounds as designers try to minimize components and reduce size, power consumption, and the like. Likewise, modulation schemes are continuously updated to reflect new approaches to maximize information transfers within limited bandwidth constraints. Changes in standards or standards based on newly available spectrum may also cause designers to approach modulator designing with different techniques.
Many different standards and modulation schemes exist, but one of the most prevalently used in the world of mobile terminals is the Global System for Mobile Communications (GSM). GSM comes in many flavors, not the least of which is General Packet Radio Services (GPRS). GPRS is a new non-voice value-added service that allows information to be sent and received across a mobile telephone network. It supplements today's Circuit Switched Data and Short Message Service. GSM allows many different types of mobile terminals, such as cellular phones, pagers, wireless modem adapted laptops, and the like, to communicate wirelessly through the Public Land Mobile Network (PLMN) to the Public Switched Telephone Network (PSTN).
One relatively recent change has been the advent of the Enhanced Data for GSM Evolution (EDGE) scheme in GSM systems. This system contains amplitude modulation components, and, as a result, the power amplifier must be linear and should not operate in saturation when classical modulation techniques are employed. Such a linear system lacks the efficiency of one that operates the power amplifier in saturation.
If a polar modulation system is used instead of a classical modulation system, then the power amplifier may operate in saturation and efficiency is greatly improved. In addition, if the polar signals are generated by a digital method, such a system does not require the use of a quadrature modulator. Quadrature modulators are undesirable from a design standpoint in that they draw large amounts of current, and hence, drain batteries comparatively fast.
In a polar modulation system, the amplitude signal that controls the power amplifier will cause unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called amplitude to phase (AM to PM) distortion, and it degrades the spectral purity of the system and the Error Vector Magnitude. Thus, a need also exists to be able to compensate for the unwanted AM to PM distortion of the transmitted phase signal.
One solution used to compensate for AM to PM distortion is to measure the AM to PM distortion of the power amplifier, and then create a polynomial for phase predistortion that is used to distort a signal prior to amplification by the power amplifier. This predistortion offsets the AM to PM distortion of the power amplifier. However, the polynomial is often at least a third order polynomial and is costly to implement. Thus, there remains a need for a more cost effective system for correcting AM to PM distortion of a power amplifier.